1. Field of the Invention
The present invention relates to a maximum-aperture regulating apparatus in which a maximum aperture size of a shutter blade is changed in accordance with a focal length of a taking lens.
2. Description of the Related Art
It is known that a low-priced shutter mechanism (program shutter) which is also used as a diaphragm employs a motor of a moving-magnet type. As to this motor, a rotor of a permanent magnet is reciprocated within a range of a predetermined angle in response to a current direction flowing in a coil. Upon this, a shutter blade is opened and closed. Although the motor of the moving-magnet type is inexpensive in comparison with a pulse motor, it is impossible to accurately control an aperture size of the shutter blade in accordance with an exposure value. For this, a regulating device is provided for regulating a rotational range of the shutter blade to a diameter at which the whole of a shutter opening is bared. An exposure amount is controlled by managing an operation time of the shutter blade.
There is a system in which an operation time of a shutter blade is controlled in accordance with an exposure value. In this system, when a taking lens is a zoom lens for example, brightness of the taking lens changes at a wide-angle side and at a telephoto side. Thus, it is necessary to make an aperture size of the shutter blade smaller at the wide-angle side in comparison with the telephoto side. Moreover, it is desirable to make a stop smaller at the wide-angle side in order to reduce a image periphery blur caused by spherical aberration of the lens. In view of this, Japanese Patent Laid-Open Publication No. 3-107132 proposes a regulating device for a zoom lens stop in which a maximum aperture size of a shutter blade, which is also used as a diaphragm, is changed in accordance with a focal length of a taking lens.
Regarding the device described in the above-mentioned Publication, a program shutter which is used as a diaphragm is incorporated in a lens group moving inside a lens barrel along an optical axis direction. From this program shutter, a part of a drive lever for driving the shutter blade projects toward an outer periphery. When zooming is performed within a prescribed range of the wide-angle side, the part of the drive lever engages with a cam to regulate a rotational range of the drive lever. The cam is provided on an inner surface of the lens barrel so as to incline in the optical axis direction. In this way, the maximum aperture size of the shutter blade is regulated in accordance with the focal length of the taking lens within the prescribed range.
However, in the device described in the above-mentioned Publication, the cam inclining in the optical axis direction is provided on the inner surface of the lens barrel so that the cam requires a length corresponding to a moving amount of the lens group. This lens group moves in the optical axis direction within the prescribed range of the wide-angle side. In case the moving amount of the lens group is long, the cam becomes long as well. Thus, there arises a problem in that the lens barrel has a large size in itself. Contrary, in case the moving amount of the lens group is small, it is necessary to steeply incline the cam. In this case, it is difficult to accurately regulate the maximum aperture size of the shutter blade.